A mayenite type compound has a typical composition of 12CaO.7Al2O3 (hereinafter referred to as “C12A7”) and a characteristic crystal structure composed of three-dimensionally linked voids (cages) with a diameter of about 0.4 nm. The framework of the cages has a positive electric charge and there are 12 cages per unit lattice. One sixth of the cages are occupied by oxygen ions in order to satisfy an electrically neutral condition of the crystal, and these oxygen ions are particularly called “free oxygen ions” because they have properties chemically different from those of the other oxygen ions constituting the framework. For the reason described above, the C12A7 crystal is denoted as [Ca24Al28O64]4+.2O2− (Non-patent Document 1).
Another known mayenite type compound is 12SrO.7Al2O3 (hereinafter referred to as “S12A7”), and there also exists a mixed crystal compound of C12A7 and S12A7 with any mixing ratio of Ca and Sr (Non-patent Document 2).
Hosono et al. found that powder of the C12A7 crystal or its sintered product was heat-treated in an H2 atmosphere to make clathrate H− ions in the cages, and then irradiated with ultraviolet light to make clathrate electrons in the cages, thereby inducing permanent electroconductivity at room temperature (Patent Document 1). The clathrate electrons are weakly bound to the cages and can freely move in the crystal, whereby electroconductivity is imparted to the C12A7 crystal of the mayenite type compound. However, the electroconductive mayenite type compound obtained by this method cannot include an enough amount of clathrate electrons, so that the electroconductivity cannot be sufficient.
Hosono et al. also found that when a C12A7 single crystal was subjected to a reducing treatment with an alkali metal vapor, the free oxygen ions in the cages were replaced by electrons to obtain a single-crystal electroconductive mayenite type compound (Patent Document 1). However, this method takes a long period of time to prepare the single crystal and to conduct the reducing treatment with calcium, and it is thus difficult to industrially apply the method.
Heretofore, it was known that a glass with the C12A7 composition could be obtained by the melting and rapid quenching method which was a usual production method of glass (Non-patent Document 3), and that the glass was re-heated to crystallize, thereby preparing C12A7 of the mayenite type compound. Li et al. reported that a temperature required for re-crystallization of the C12A7 glass obtained by the melting and rapid quenching method in air was from 940 to 1,040° C.; a main crystal phase produced was the C12A7 crystal of the mayenite type compound; and a CaAl2O4 crystal was obtained as a by-product (Non-patent Document 4). The mayenite type compound thus obtained was, however, an insulator having free oxygens in the cages.
Hosono et al. found that a transparent glass prepared by melting the C12A7 crystal in a carbon crucible was subjected to a re-heating treatment at 1,600° C. and in an atmosphere with an oxygen partial pressure as extremely low as 10−11 Pa for one hour or at 1,000° C. in vacuum for 30 minutes to crystallize, thereby producing an electroconductive mayenite type compound (Non-patent Document 5). It was, however, difficult to industrially produce the compound at low cost and on a large scale by this method because the re-heating treatment required the high temperature to re-melt the glass and the atmosphere of extremely low oxygen partial pressure or vacuum, as described above.
Patent Document 1: WO2005-000741
Non-patent Document 1: F. M. Lea and C. H. Desch, The Chemistry of Cement and Concrete, 2nd ed., p. 52, Edward Arnold & Co., London, 1956.
Non-patent Document 2: O. Yamaguchi, A. Narai, K. Shimizu, J. Am. Ceram. Soc. 1986, 69, C36.
Non-patent Document 3: Minoru Imaoka, Glass Handbook (compiled by Sakka, Takahashi, Sakaino), Asakura Publishing, 880 pages (1975)
Non-patent Document 4: W. Li, B. S. Mitchell, J. Non-Cryst. Sol. 1999, 255 (2, 3), 199.
Non-patent Document 5: S. W. Kim, M. Miyakawa, K. Hayashi, T. Sakai, M. Hirano, and H. Hosono, J. Am. Chem. Soc., http://pubs.acs.org/journals/jacsat/, Web Release Date: 15 Jan. 2005).